By now you’ve heard about R.E.M.’s Mike Mills breaking the news of David Letterman’s retirement yesterday, but here’s the reason he was even there: Mills and former bandmate Peter Buck reunited on “The Late Show” to back Joseph Arthur’s performance of “Walk on the Wild Side.”

There very well could have been a full-fledged R.E.M. reunion in Portland, Ore., on Saturday night as all four members of the defunct college-rock superstars performed after Peter Buck’s wedding. But, by all appearances, they managed to avoid such headlines, just barely, by not actually sharing the stage together.

During a break from sessions for R.E.M.’s next album, Mike Mills contributed a cover of Billy Bragg’s ‘Sing Their Souls Back Home’ — off the agit-folkie’s 2008’s album ‘Mr. Love & Justice’ — to The Voice Project.